In recent years, to address environmental problems and the like, weight reductions and enhanced strength are being sought with respect to members used in automobiles, industrial machinery, buildings and the like. In particular, tensile strength of 1000 MPa or more is being demanded with respect to bolts for automobiles such as engine cylinder head bolts and connecting rod bolts.
However, if a bolt has high tensile strength of 1000 MPa or more, the susceptibility to hydrogen embrittlement increases, and hydrogen embrittlement resistance (delayed fracture) characteristics are lowered. SCM steel (JIS Standard) that contains a large amount of an alloying element such as Mo, and alloy steel that contains an expensive alloying element such as V are used as the starting material for such high-strength bolts. These alloy steels are manufactured into wire rods, and are further subjected to wire drawing and cold forging to be manufactured into bolts.
In the case of using the aforementioned alloy steels as bolts, hydrogen embrittlement resistance characteristics are enhanced. However, because these alloy steels contain alloying elements in large amounts, the hardenability increases. Consequently, when these alloy steels are hot-rolled to produce wire rods, a hard microstructure such as bainite is formed. Because a wire rod that contains a hard microstructure is hard, it is difficult to perform wire drawing and cold forging thereon. Therefore, when forming bolts using wire rods of these alloy steels, it is a common practice to carry out a softening heat treatment multiple times prior to performing wire drawing and cold forging. The multiple softening heat treatments raise the production cost of the bolt. Accordingly, there is a need for a bolt for which enhanced strength and excellent hydrogen embrittlement resistance characteristics can be realized while keeping down the production cost.
To suppress the formation of bainite when producing a wire rod, it is sufficient to reduce the amount of alloying elements such as Mo and V contained in the steel. In this case, because formation of bainite is suppressed, a softening heat treatment can be omitted or simplified. However, it becomes difficult to provide the bolt with high strength, and the hydrogen embrittlement resistance characteristics also decrease.
Bolts that have high strength are proposed, for example, in the respective Patent Literatures described hereunder. The bolts proposed in these Patent Literatures contain boron to thereby increase hardenability, strengthen grain boundaries, and increase the strength.
Specifically, a bolt disclosed in Japanese Patent Application Publication No. 10-53834 (Patent Literature 1) contains, in mass %, B: 0.0008 to 0.004%, C: 0.4% or less, Ti: 0.025 to 0.06% and N: 0.006% or less. In this bolt, the relation between the ferrite grain size FGc and Ti compounds excluding TiN during hot rolling satisfies the expression: [amount of Ti compounds excluding TiN/FGc1/2]×1000≥3. In addition, the austenite grain size number is 5 or more. It is described in Patent Literature 1 that, as a result, the tensile strength is more than 785N/mm2.
However, in the bolt disclosed in Patent Literature 1, in a case where there is a high Mn content and a low Cr content, the hydrogen embrittlement resistance characteristics are low in some cases.
A bolt disclosed in National Publication of International Patent Application No. 2009-521600 (Patent Literature 2) has a composition containing, in weight %, 0.35 to 0.55% of carbon, 0.05 to 2.0% of silicon, 0.1 to 0.8% of manganese, 0.001 to 0.004% of boron, 0.3 to 1.5% of chromium, 0.005% or less of total oxygen (T.O.), 0.015% or less of phosphorus and 0.010% or less of sulfur, and also containing at least one type of element selected from the group consisting of 0.05 to 0.5% of vanadium, 0.05 to 0.5% of niobium, 0.1 to 0.5% of nickel, 0.1 to 1.5% of molybdenum and 0.01 to 0.1% of titanium, with the balance being Fe and impurities. This bolt has an internal micro-structure made of ferrite and tempered martensite, and the ferrite content in the internal micro-structure is 3 to 10% by area fraction. It is described in Patent Literature 2 that this bolt is excellent in delayed fracture resistance characteristics and has enhanced strength.
However, the bolt proposed in Patent Literature 2 is made of dual phase steel having soft ferrite in an amount of 3 to 10% by area fraction and tempered martensite as the internal micro-structure of the bolt. Consequently, the bolt strength is apt to decrease in comparison to the case of steel having a tempered martensite single-phase structure. Therefore, in order to adjust the strength of the steel to a desired strength level, it is necessary to perform tempering at a lower temperature in comparison to steel that has a tempered martensite single-phase structure. Consequently, in some cases the hydrogen embrittlement resistance characteristics decrease at the desired strength. In addition, it is necessary to carry out treatment to adjust the ferritic microstructure, such as re-quenching and tempering during the production process. Consequently, the production cost increases.
A high-strength bolt disclosed in Japanese Patent Application Publication No. 2008-156678 (Patent Literature 3) is made from steel that contains, in mass %, C: more than 0.15% to 0.30% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0.1% or less, Mo: 0.3% or more to 0.5% or less and B: 0.0005% or more to 0.01% or less, with the balance being Fe and impurities. The steel is quenched, and thereafter subjected to tempering at 100 to 400° C. and the steel microstructure is made into a microstructure in which the average prior-austenite grain size after quenching is 10 μm or less. It is described in Patent Literature 3 that, by this means, a high-strength bolt that has the bolt strength range from approximately 1200 to 1800 MPa and has excellent delayed fracture resistance characteristics and corrosion resistance can be obtained.
However, because the bolt disclosed in Patent Literature 3 contains 0.3 to 0.5% of Mo, the hardenability is too high. Therefore, it is necessary to carry out a softening heat treatment for an extended period of time before performing wire drawing and cold forging. In such a case, the hydrogen embrittlement resistance characteristics may sometimes decrease.
Steel for a high-strength bolt that is disclosed in Japanese Patent Application Publication No. 2012-162798 (Patent Literature 4) contains, in mass %, C: 0.20 to less than 0.40%, Si: 0.20 to 1.50%, Mn: 0.30 to 2.0%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Ni: 0.05 to 1.0%, Cr: 0.01 to 1.50%, Cu: 1.0% or less (including 0%), Al: 0.01 to 0.10%, Ti: 0.01 to 0.1%, B: 0.0003 to 0.0050% and N: 0.002 to 0.010%, in which one or more types of element selected from the group consisting of Cu, Ni and Cr are contained in a total amount of 0.10 to 3.0%, with the balance being Fe and unavoidable impurities. In the steel, a ratio ([Si]/[C]) between the Si content [Si] and the C content [C] is 1.0 or more. It is described in Patent Literature 4 that by this means, without adding a large amount of an expensive alloying element such as Cr or Mo, a boron-added high-strength bolt that is excellent in delayed fracture resistance while also having high strength of 1100 MPa or more can be obtained.
However, in the steel disclosed in Patent Literature 4, the Ni content is high. Therefore, in some cases the hardenability is too high. Consequently, it is necessary to carry out a softening heat treatment for an extended period of time before performing wire drawing and cold forging. In such a case, the hydrogen embrittlement resistance characteristics sometimes decrease.
Steel for cold forging that is disclosed in Japanese Patent Application Publication No. 11-92868 (Patent Literature 5) contains, in mass %, C: 0.10 to 0.40%, Si: 0.15% or less, Mn: 0.30 to 1.00%, Cr: 0.50 to 1.20%, B: 0.0003 to 0.0050% and Ti: 0.020 to 0.100%, in which the content of P is limited to 0.015% or less (including 0%), the content of S is limited to 0.015% or less (including 0%), and the content of N is limited to 0.0100% or less (including 0%), with the balance being Fe and unavoidable impurities. In addition, the total quantity of particles of one or more types among TiC and Ti(CN) having a diameter of 0.2 μm or less in the matrix of the steel is 20 particles/100 μm2 or more. It is described in Patent Literature 5 that, by this means, coarsening of the grains is prevented and delayed fracture resistance characteristics can be improved.
However, the bolt disclosed in Patent Literature 5 is not specialized technology, and in the case where bolts are produced, the hydrogen embrittlement resistance characteristics may be low in some cases.